At the main menu, select the "Top Secret" option. Enter one of the following codes, represented by the Numbuh of the kids to activate the corresponding cheat function:

Invincibility

Enter 432513 as a code.

Super Special in Spankarific

When you see a soccer ball goal, kick the soccer ball into it to get a Super Special.

Super Secret in Yatta Yatta Yatta stage

While playing as Numbuh 5 on the Yatta Yatta Yatta stage, get to the part with the disco ball. Note: When you step under the disco light, robots will appear. When you get the first intermission sequence, you will see you are entirely surrounded. Near the left in the far distance should be a pool table. Go over to it. Once there, notice the pool balls on the table. Use your character to push the balls into the nearby fireplace. Once you get one in, you should hear the same sound that plays when you get a rainbow monkey. Keep track of all the pool balls. If you lose one and cannot find it, you cannot get the Super Secret. When you get all the pool balls in the fireplace, an icon will drop from it. Run into it and you should get a Super Secret unlocked message.